1. Field of the Invention
The present invention relates to a distance measuring device based on laser interference, which is used to perform distance measurement with high precision.
2. Related Background Art
A distance measuring device based on laser interference uses a laser source (coherent light beam) having a long coherence length and measures a distance by using interference of light. In general, this device has high measurement precision and hence has been used for high-precision distance measurement in various cases.
In an exposure apparatus (so-called stepper or the like) used for the manufacture of semiconductor elements, before exposure is performed, a circuit pattern (its projection image) on a mask and a circuit pattern on a semiconductor wafer must be superimposed on each other with high precision. This superimposition is generally performed by moving a moving stage holding the semiconductor wafer with respect to the mask. In order to increase the superimposition precision, therefore, it is important to position the moving stage with high precision.
Such high-precision positioning (or position measurement) of the moving stage is performed by using a distance measuring device based on laser interference. In general, an object such as a reflecting mirror is fixed on one end of the moving stage, and a laser beam from the light source is irradiated on the object to measure its position.
When a device based on double-beam interference is to be used, a light beam from a light source is split into a reference beam and a measurement beam by a beam splitter (half mirror). The measurement beam is irradiated on the object on the moving stage, and is returned to the beam splitter. The measurement beam is then caused to interfere with the reference beam having propagated through another optical path. The resultant pattern of interference fringes corresponds to the optical path length difference (phase difference) between the two light beams.
When, therefore, the position of the moving stage, i.e., the position of the object, changes, the optical path length difference between the two light beams fluctuates. The pattern of interference fringes therefore changes. According to the principle of interference, every time the position of the moving state moves by a distance corresponding to 1/2 the wavelength of a light beam, the dark and light portions of the interference fringes change to light and dark portions, respectively. Position measurement can be performed by obtaining the number of changes in the light and dark portions of the interference fringes.
The positioning precisions of currently available distance measuring devices based on laser interference are about 10 nm. However, with demands for higher LSI integration degrees, a further increase in precision is required.